what exactly ARE dementors??

[Samantha]

No: HPFGUIDX 136040

Hi Potterphiles!
 There are many, many descriptions of how dementors make you feel
(ie:cold, like you will never be happy again) and what they do(give 
you the "kiss", suck out your soul. BUT, what ARE dementors? Are they 
wizard made?(to guard Azkaban) or are they like hags, trolls and 
goblins? Seperate creatures unto themselves? I have read each book 
except the last one four or five times, and I don't remember them ever 
saying how dementors came about.

Snapeo'phile